Gas quenching method

ABSTRACT

A gas quenching method of the present invention includes a first stage (t1 to t2) at which a workpiece is subjected to a rapid cooling by forcibly circulating a cooling gas, a second stage (t2 to t3) at which the circulation of the cooling gas is stopped and pressure is reduced inside the furnace to conduct heat insulation, and a third stage (as from t3) at which the workpiece is cooled again by the cooling gas. At the second stage, the workpiece is maintained at an intermediate temperature that is higher than martensite transformation start temperature, and, during this, temperature throughout the workpiece is made uniform. Therefore, it is possible to achieve a uniform quenching and suppress distortion caused by difference of the cooling speed.

TECHNICAL FIELD

This invention relates to a gas quenching method in which a workpiece is heated and then cooled by using a cooling gas, as a quenching of steel.

BACKGROUND TECHNOLOGY

Quenching of steel is a heat treatment technology to obtain a martensite structure by turning steel into a high-temperature condition and then rapid cooling. Hitherto, there has been adopted many times a liquid quenching method in which cooling after heating is conducted by using, as a cooling agent, a liquid, such as oil, water or a polymer solution, which is relatively high in cooling property to conduct quenching of relatively large parts. In this liquid quenching, however, boiling occurs non-uniformly during quenching. As a result, the cooling speed becomes non-uniform, thereby making quality unstable. Furthermore, it is necessary to have a washing step for removing the cooling agent after quenching, and a waste water treatment resulting from the washing also becomes a major problem.

From such point, in recent years, attention has been attracted to a gas quenching in which an inert gas, such as nitrogen gas, is used as the cooling agent, and the cooling gas is allowed to flow, for example, around workpieces arranged in a furnace, thereby conducting rapid cooling or quenching of the workpieces.

Furthermore, Non-patent Publication 1 discloses, as a type of the gas quenching method, an isothermal quenching (also called multi-stage quenching) in which an isothermal maintenance is conducted for a certain period of time in the middle of the cooling by using a hot gas of a high temperature of around 300° C. In this method, the cooling gas is previously heated to around 300° C. by using factory exhaust heat or the like, and this hot gas is circulated through a gas furnace that accommodates workpieces heated to around 1000° C., thereby cooling the workpieces and conducting an isothermal treatment on the workpieces to a temperature of around 300° C. that is in equilibrium with the temperature of the hot gas. Then, after the temperature equilibrium, it is switched to circulation of the cooling gas having low temperatures by passing through a cooler, thereby cooling the workpieces to complete quenching.

It is described in Non-patent Publication 1 that distortion of the workpiece is reduced by conducting such a multi-stage quenching, as compared with a normal continuous quenching.

However, in a conventional method to achieve the multi-stage quenching by using a plurality of gases having different temperatures like Non-Patent Publication 1, it becomes necessary to provide the gas furnace with a heat exchanger for heating gas, a cooler for cooling gas, a damper for switching the passage, and so on. This makes the facility complicated.

Furthermore, it is aimed to obtain an isothermal condition by an equilibrium between temperature of the hot gas and temperature of the workpiece. Therefore, it takes time during which temperature of the workpiece reaches the target isothermal treatment temperature, and the cycle time of the quenching treatment as a whole becomes long.

PRIOR ART PUBLICATIONS Non-Patent Publications

-   Non-patent Publication 1: Akihiro HAMABE, “Using Preheated Inactive     Gas for Vacuum Hardening and Isothermal Heat Treatment after     Carburizing”, Journal of the Vacuum Society of Japan, 2010, Vol. 53,     No. 1, pages 49-52.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a gas quenching method in which a workpiece made of steel is heated and then cooled for quenching by allowing a cooling gas to flow around the workpiece in a furnace, the gas quenching method comprising:

stopping supply of the cooling gas in the middle of the quenching before the workpiece reaches a martensite transformation start temperature;

reducing pressure inside the furnace and making temperature throughout the workpiece uniform by radiation cooling, while temperature of the workpiece is maintained at an intermediate temperature that is higher than the martensite transformation start temperature; and

resuming supply of the cooling gas after the temperature throughout the workpiece has been made uniform, thereby conducting the quenching to pass the martensite transformation start temperature.

That is, in the quenching method of the present invention, in the middle of a quenching using a cooling gas, supply of the cooling gas is stopped, and pressure inside the furnace is reduced to suppress cooling speed of the workpiece. In particular, the cooling action by convection is rapidly suppressed by reducing pressure inside the furnace, resulting in substantially only radiation cooling. In other words, the furnace turns into a heat insulated condition by the pressure reduction, such that the workpiece is temporarily maintained at the intermediate temperature. At this time, heat transfers in the workpiece from a relatively high-temperature site to a relatively low-temperature site, thereby making the temperature throughout the workpiece uniform. Therefore, at the subsequent cooling by supplying the cooling gas, temperatures throughout the workpiece pass the martensite transformation start temperature almost at the same time and with similar temperature gradients. Thus, the quenching is conducted more uniformly.

According to the present invention, it is possible to achieve a multi-stage quenching without necessity of a plurality of gases with different temperatures, and distortion of the workpiece resulting from quenching is reduced by making the temperature throughout the workpiece uniform. Furthermore, as compared with a conventional method using a hot gas, it is possible to conduct the cooling and the isothermal treatment until the intermediate temperature within a short period of time, thereby shortening the cycle time of the quenching treatment as a whole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a structure of a gas quenching furnace used in the gas quenching method of the present invention;

FIG. 2 is an explanatory view showing steps of the gas quenching method of Example;

FIG. 3 is a perspective view showing one example of the workpiece;

FIG. 4 is a perspective view of a lower link as a whole to become the workpiece; and

FIG. 5 is a characteristic diagram showing a comparison between Example and Comparative Example in the amount of distortion resulting from the quenching.

MODE FOR IMPLEMENTING THE INVENTION

In the following, an embodiment of the present invention is explained in detail.

FIG. 1 shows one example of gas quenching furnace 1 used in the gas quenching method of the present invention. This gas quenching furnace 1 is a vertical furnace with an elliptical shape that is elongated in vertical direction when viewed from the front. It is formed at its upper part with fan 2 that circulates the cooling gas in gas quenching furnace 1 and stirs the cooling gas. At its lower part, there is disposed one-stage or multi-stage tray 3 on which a plurality of the after-mentioned workpieces as the targets of the quenching treatment are arranged. This tray 3 has a latticed structure having many openings such that flow of the cooling gas (shown by arrow G in the drawing) sent by fan 2 is allowed to pass through the tray 3 and then flow in an upward direction. This tray 3 is taken into and out of the furnace through a door not shown in the drawings.

Gas quenching furnace 1 has a sealed structure that is resistant against a predetermined depressurized condition, and is equipped outside with depressurization pump 4 for depressurizing the furnace. This depressurization pump 4 is connected to the space inside the furnace through depressurization passage 5, and depressurization passage 5 is equipped with on-off valve 6 with solenoid valve, etc.

Furthermore, gas quenching furnace 1 is equipped with gas introducing passage 7 for introducing a cooling gas, such as nitrogen gas, hydrogen gas, helium gas or argon gas, into the furnace, and gas discharging passage 9 for discharging the cooling gas from the furnace. Gas introducing passage 7 is equipped with on-off valve 8 with solenoid valve, etc. Gas discharging passage 9 is similarly equipped with on-off valve 10 with solenoid valve, etc.

FIG. 2 shows an embodiment of the gas quenching method of the present invention using the above-mentioned gas quenching furnace 1. A workpiece used in this embodiment is one prepared by machining chromium steel of SCr420 as base material into a predetermined shape and then previously conducting a carburizing treatment on the surface by gas carburizing. The target carbon concentration of the surface in the carburizing treatment is 0.6%. Therefore, the material on the surface of the workpiece is one equivalent to SCr460. The carburizing treatment is conducted in another furnace. After annealing from the carburizing treatment temperature, it is introduced together with tray 3 into gas quenching furnace 1 in a condition where it has been subjected to a reheating until 1050° C. for quenching.

After closing the door (not shown in the drawings) of gas quenching furnace 1, the cooling gas is introduced into gas quenching furnace 1 through gas introducing passage 7. Once filled with the cooling gas, the inside of gas quenching furnace 1 is turned into a sealed condition by closing on-off valve 8, etc. Then, fan 2 is driven to cool the workpiece by forcibly circulating the cooling gas. As the cooling gas, for example, nitrogen gas having a temperature adjusted to 40° C. is used.

FIG. 2(a) shows temperature change of the workpiece, FIG. 2(b) shows an on-off condition of the gas cooling or fan 2, and FIG. 2(c) shows an on-off condition of depressurization of the furnace or depressurization pump 4. From time t1, the workpiece is rapidly cooled by forcibly circulating the cooling gas. With this, temperature of the workpiece is abruptly lowered. FIG. 2(a) also shows a bainite transformation curve (B) where transformation into bainite occurs resulting from the cooling prior to martensite transformation, but the speed of the temperature lowering by the cooling gas is set not to pass this nose-shape bainite transformation curve.

Following such rapid cooling period, before temperature of the workpiece reaches the martensite transformation start temperature, fan 2 is stopped at time t2 to stop circulation and stirring of the cooling gas. At substantially the same time as this, depressurization pump 4 is energized to depressurize the inside of gas quenching furnace 1. By stopping fan 2, cooling by the cooling gas is suppressed. However, the inside of gas quenching furnace 1 turns into a thermally insulated condition by depressurizing the inside of gas quenching furnace 1. That is, the cooling action by convection is rapidly suppressed, resulting in slightly only radiation cooling by radiation from the surface of the workpiece. With this, the cooling speed of the workpiece becomes very small, and temperature of the workpiece is temporarily maintained at an intermediate temperature that is higher than martensite transformation start temperature, as shown in FIG. 2(a). The target intermediate temperature is, for example, 300° C., which is slightly higher than martensite transformation start temperature (Ms).

During the rapid cooling period between times t1 to t2, there are some differences in cooling speed throughout the workpiece. As shown by solid line F in FIG. 2(a), the temperature lowering progresses early at a site with a rapid cooling speed. In contrast, as shown by broken line L, the progress of the temperature lowering becomes slow at a site with a relatively slow cooling speed. Therefore, at time t2, there occur temperature differences among the sites. While the workpiece is substantially in a heat insulated condition by stopping fan 2 and the depressurization, heat transfers from a relatively high-temperature site to a relatively low-temperature site, and an isothermal condition is obtained throughout the workpiece at the target intermediate temperature (e.g., 300° C.) which is slightly higher than martensite transformation start temperature. That is, temperature shown by solid line F and temperature shown broken line L of FIG. 2(a) converge and are maintained at around 300° C.

Herein, to control stopping of fan 2 and turning-on of depressurization pump 4, it is optional to monitor the actual temperature of the workpiece by using, for example, an infrared-type temperature sensor, etc. and to execute stopping of fan 2 and turning-on of depressurization pump 4 when becoming a predetermined temperature that is slightly higher than the target intermediate temperature in an isothermal condition in view of the delay of temperature change. Alternatively, it is optional to experimentally determine the necessary time in which the temperature lowers to a predetermined temperature from time t1 and then to execute stopping of fan 2 and turning-on of depressurization pump 4 when the elapsed time from time t1 has reached the predetermined value. In one embodiment, the initial rapid cooling period from time t1 to time t2 is, for example, about 45 seconds.

Once completing an isothermal condition throughout the workpiece by maintaining the intermediate temperature, at time t3, depressurization pump 4 is turned off, the cooling gas is reintroduced into gas quenching furnace 1 through gas introducing passage 7, and fan 2 is driven to restart rapid cooling of the workpiece by forcibly circulating the cooling gas. The cooling gas may be the same one as that of the initial rapid cooling period. For example, there is used a nitrogen gas of which temperature has been adjusted to 40° C.

By the above rapid cooling, temperature of the workpiece lowers to cross martensite transformation start temperature (Ms) (that is, pass martensite transformation start temperature (Ms)) to conduct quenching. At this time, an isothermal condition is achieved throughout the workpiece. Thus, throughout the workpiece, timing and temperature gradient (cooling speed) when passing martensite transformation start temperature become constant. Therefore, martensite transformation occurs evenly therethroughout to obtain an even quenching.

The necessary time from time t2 to time t3 is, for example, about 30 seconds in one embodiment. To control restarting of the cooling at time t3, it suffices to experimentally determine the time necessary for an isothermal condition and to restart cooling when the elapsed time from time t2 has reached a predetermined value. Alternatively, it is optional to monitor the actual temperatures of a plurality of sites of the workpiece by using an infrared-type temperature sensor, etc. and to restart cooling when these have converged on generally the same temperature.

Cooling as from time t3 is conducted, for example, for about 2 to 5 minutes in one embodiment.

In this way, in the quenching method of the above-mentioned embodiment, as a gas quenching using a single cooling gas, there is achieved a multi-stage quenching including the first stage of a rapid cooling period between time t1 and time t2, the second stage of an isothermal period between time t2 and time t3, and the third stage of a rapid cooling period as from time t3. In this way, by having the second stage as a period for obtaining an isothermal condition at the intermediate temperature which is slightly higher than martensite transformation start temperature, it is possible to conduct a uniform quenching with a small distortion resulting from the quenching. Furthermore, it is possible in the second stage to rapidly lower the cooling speed by using heat insulation by the depressurization. Therefore, the necessary time of the first stage and the second stage becomes short. Thus, for example, as compared with a conventional method of using a hot gas, the cycle time becomes shorter.

Herein, as shown in FIG. 2(a), the temperature of the second stage between time t2 and time t3 is set at a temperature that is higher than martensite transformation start temperature (Ms) and is lower than the nose-shape bainite transformation curve. That is, the intermediate temperature and the period of the second stage are set such that the characteristic of temperature change of the workpiece does not cross the bainite transformation curve. With this, transformation into bainite during quenching is suppressed.

FIG. 3 shows one example of the workpiece suitable for the quenching method of the present invention. This workpiece is a component constituting a part of lower link 11 (see FIG. 4) in a multi-link type piston crank mechanism of an internal combustion engine. As described in, for example, Japanese Patent Application Publication 2015-42849, this type of lower link 11 is one for connecting an upper link with one end connected to a piston pin and a crank pin of a crankshaft. As shown in FIG. 4, it is formed at its center with a cylindrical crank pin bearing portion 12 to be fitted onto the crank pin. Furthermore, it is provided with a pin boss portion 13 for an upper pin and a pin boss portion 14 for a control pin at positions on opposite sides by almost 180 degrees with an interposal of this crank pin bearing portion 12. This lower link 11 as a whole forms a parallelogram close to rhombus. On division surface 15 passing through center of crank pin bearing portion 12, it is formed of two divided parts of lower link upper 11A containing the pin boss portion 13 for upper pin and lower link lower 11B containing the pin boss portion 14 for control pin. The workpiece of the above embodiment is the above-mentioned lower link upper 11A.

Pin boss portion 13 for upper pin in this lower link upper 11A has a bifurcated structure to sandwich the upper link at its center portion in the axial direction. That is, it is formed into a pair of wall-like ones opposite to each other with an interposal of a center recess portion 16.

This workpiece, that is, lower link upper 11A, is disposed on the above-mentioned tray 3 with a posture shown in FIG. 3. That is, it is retained to have an upright posture in which one side surface 17 (see FIG. 4) perpendicular to division surface 15 becomes a bottom surface that is brought into contact with tray 3 and in which division surface 15 stand upright from tray 3. Then, the cooling gas is guided to be parallel with division surface 15 in gas quenching furnace 1, and the cooling gas is allowed to flow along the front and back surfaces of a pair of wall-like pin boss portions 13.

In quenching against such workpiece, wall-like pin boss portion 13 has a thinner thickness as compared with a part in the vicinity of division surface 15 and is widely exposed to the gas flow. Therefore, in general, wall-like pin boss portion 13 becomes a portion with a rapid cooling speed, and a thick portion in the vicinity of division surface 15 becomes a portion with a slow cooling speed. Furthermore, an outer surface and an inner surface (the surface on the side of recess portion 16) of wall-like pin boss portion 13 are different in cooling speed. As a result, as quenching progresses, it tends to have a distortion in which wall-like pin boss portion 13 is displaced in the axial direction of lower link 11.

According to the multi-stage quenching method of the above embodiment, it is possible to suppress distortion of such wall-like pin boss portion 13 in the axial direction.

FIG. 5 shows results of comparative experiments in the case of the multi-stage quenching method of Example and in the case of a simple continuous quenching to continue cooling by the cooling gas as Comparative Example, in terms of change of the distance between the pair of pin boss portions 13 (in other words, the width of the recess portion 16 in the axial direction) due to the above distortion. Herein, in quenching of Example, as the first stage, nitrogen gas of 40° C. was introduced under a pressure of 0.6 MPa, and it was circulated by fan 2, thereby conducting a rapid cooling for 1 minute. Then, as the second stage, it was depressurized to 1 kPa, followed by maintaining for 30 seconds. Furthermore, as the third stage, nitrogen gas of 40° C. was introduced under a pressure of 0.6 MPa, and it was circulated by fan 2, thereby conducting a cooling for 1 minute. In Comparative Example, nitrogen gas of 40° C. was introduced under a pressure of 0.6 MPa, and it was circulated by fan 2, thereby conducting a cooling for two minutes and thirty seconds.

As shown in the drawing, according to the multi-stage quenching of Example, as compared with the continuous quenching, there was obtained a result that distortion of pin boss portion 13 in the axial direction was reduced by half.

As above, one embodiment of the present invention was explained, but the present invention is not limited to the above embodiment. Various modifications are possible, including the treatment temperature, time, etc. Furthermore, the present invention is also suitable for quenching of lower link lower 11B of lower link 11 shown in FIG. 4 and can be applied to quenching of other various parts. 

1. A gas quenching method in which a workpiece made of steel is heated and then cooled for quenching by allowing a cooling gas to flow around the workpiece in a furnace, the gas quenching method comprising: stopping supply of the cooling gas in the middle of the quenching before the workpiece reaches a martensite transformation start temperature; reducing pressure inside the furnace and making temperature throughout the workpiece uniform by radiation cooling, while temperature of the workpiece is maintained at an intermediate temperature that is higher than the martensite transformation start temperature; and resuming supply of the cooling gas after the temperature throughout the workpiece has been made uniform, thereby conducting the quenching to pass the martensite transformation start temperature.
 2. The gas quenching method as claimed in claim 1, wherein the temperature throughout the workpiece is made uniform, while the temperature of the workpiece is maintained at a temperature that is higher than the martensite transformation start temperature and lower than a bainite transformation curve.
 3. The gas quenching method as claimed in claim 1, wherein the workpiece has a surface that is previously subjected to a carburizing treatment.
 4. A gas quenching method, comprising: a first step of subjecting a workpiece made of steel to a rapid cooling in a furnace by a cooling gas from a heated condition; a second step of stopping supply of the cooling gas to the workpiece and reducing pressure inside the furnace, such that, in the middle of a temperature lowering of the workpiece, the workpiece is maintained at an intermediate temperature that is higher than a martensite transformation start temperature; and a third step of conducting a rapid cooling again by the cooling gas after temperature of the workpiece has been made uniform. 